Electrostatic discharge (ESD) and electro-over stress (EOS) are significant causes of semiconductor device failures at all stages of manufacturing, assembly and test. Damages from ESD or EOS can cause either complete device failure or device weakness by parametric shift, locally heating, or otherwise by damaging oxides, junctions or device components. A parasitic bipolar junction transistor (BJT) essentially formed below a MOS transistor is often applied to conduct the sudden flow of electricity caused by an ESD/EOS event, so as to provide an ESD/EOS protection function.
However, an ESD/EOS protection performance of the BJT is often influenced by a change of carrier concentration profile of the components in the BJT. The change of carrier concentration profile can be resulted from a thermal treatment applied during a wafer manufacturing process. As a consequence of the change of carrier concentration profile of the BJT, an electrical characteristic and an ESD/EOS protection ability of the BJT is altered.